Replace hasAttr/getAttr calls with the ? and . operators

For NixOS evaluation, this gives a ~21% reduction in the number of
values allocated and a ~4% speedup. It's also more readable.

nixos/modules/system/boot/systemd-unit-options.nix

@@ -6,8 +6,8 @@ let
 
   checkService = v:
     let assertValueOneOf = name: values: attr:
-          let val = getAttr name attr;
-          in optional ( hasAttr name attr && !elem val values) "Systemd service field `${name}' cannot have value `${val}'.";
+          let val = attr.${name};
+          in optional (attr ? ${name} && !elem val values) "Systemd service field `${name}' cannot have value `${val}'.";
         checkType = assertValueOneOf "Type" ["simple" "forking" "oneshot" "dbus" "notify" "idle"];
         checkRestart = assertValueOneOf "Restart" ["no" "on-success" "on-failure" "on-abort" "always"];
         errors = concatMap (c: c v) [checkType checkRestart];

nixos/modules/system/boot/systemd.nix

@@ -321,7 +321,7 @@ let
           [Service]
           ${let env = cfg.globalEnvironment // def.environment;
             in concatMapStrings (n:
-              let s = "Environment=\"${n}=${getAttr n env}\"\n";
+              let s = "Environment=\"${n}=${env.${n}}\"\n";
               in if stringLength s >= 2048 then throw "The value of the environment variable ‘${n}’ in systemd service ‘${name}.service’ is too long." else s) (attrNames env)}
           ${if def.reloadIfChanged then ''
             X-ReloadIfChanged=true